Device and method for calibrating a sensor

ABSTRACT

In order to counteract age-related distortion of measured values, the present invention proposes a device and a method for calibrating a sensor in general and a force sensor and a belt-force sensor ( 5 ), in particular, which allow reliable calibration of the sensor in a simple manner.  
     The device according to the present invention for calibrating a sensor includes means for detecting at least one defined state of the sensor. Only when the sensor is in a defined state thusly detected, a corresponding adjustment of the measured value of the sensor is implemented within the framework of the method according to the present invention.

BACKGROUND INFORMATION

[0001] The present invention is directed to a device and a method for calibrating a sensor in general and a force sensor and a belt-force sensor, in particular.

[0002] The present invention is described in the following on the basis of a plurality of sensors from the automotive sector. However, it is not restricted to these sensors, but may also be realized in connection with sensors from other fields of application.

[0003] The triggering of restraining means in a motor vehicle usually occurs as a function of various parameters such as the severity of the impact and the position and the weight of the passengers. In this context, it is useful to take the respective belt force into consideration as well, which is explained in greater detail in the following on the basis of an example.

[0004] For the triggering of a multi-stage airbag and for the triggering of belt tighteners, it is known to acquire data regarding the profile of the individual passengers, which allows conclusions as to their weight. For this purpose, a force sensor, or a force-sensor array, is normally used, which is installed in the seat and realized, for example, in the form of a sensor mat having pressure-sensitive sensor cells. The sensor cells are arranged in a matrix and may be triggered individually, so that it is possible to detect the pressure distribution in the seat in the form of a value matrix. The heavier a person, the more heavily the seat material is compressed and the higher the number of activated sensor cells. Of course, the pressure value detected by a sensor cell is per se also dependent on the respective pressurization. In evaluating the pressure data detected by the individual sensor cells of the sensor mat, the number of the activated sensor cells is therefore taken into consideration as well, in addition to the individual pressure values. A control device, which is also installed in the seat, carries out the evaluation. The result of the evaluation is transmitted to the air bag control device, so that the individual seat occupancy may be taken into consideration in the triggering of the restraining means. This method is largely independent of external conditions such as cold or wear. Nevertheless, the force-sensor array should be calibrated at regular time intervals in order to ensure constant measuring results at a given weight force.

[0005] A child seat may be detected on the basis of its seat profile, which does not resemble that of a person. However, if a child seat is strapped in with a strong belt force of approximately 150 N, it is pressed into the seat with such force that its seat profile is distorted and it appears heavier than it actually is.

[0006] However, a belt force of more than 30 N cannot really be applied over a long period of time when a person is strapped in since the belt usually yields. Such a high belt force would also be so uncomfortable that no passenger would maintain it. Therefore, when the belt force exceeds a certain threshold value it can be assumed with a high degree of certainty that it is not a person that is strapped in but a child seat or some other object. On the basis of the data of the applied belt force, it is thus already possible to implement an occupancy detection to a certain extent.

[0007] A prerequisite for being able to consider the force exerted by the belt when evaluating the pressure data is the use of a belt-force sensor. However, for this purpose it is necessary to ensure that a measuring value of the belt-force sensor is constant in the long term or over the service life of the motor vehicle at a given belt force. This is problematic in practice, in particular in static belt-force sensors, since it is often the case here that time drift occurs in the measured values.

SUMMARY OF THE INVENTION

[0008] In order to counteract age-related distortion of the measured values, the present invention proposes a device and a method for calibrating a sensor in general and a force sensor and a belt-force sensor, in particular, which allow reliable calibration of the sensor in a simple manner.

[0009] To this end, the device according to the present invention includes means for detecting at least one defined state of the sensor. Within the framework of the method according to the present invention, a correction of the measured value of the sensor is implemented only when the sensor is in a defined state, that is, the measured value of the sensor corresponding to this state is known.

[0010] It is particularly advantageous that the device and the method according to the present invention are able to be used regardless of the technical design of the sensor.

[0011] For instance, according to the present invention, a dynamically sensing acceleration sensor or also a low-g acceleration sensor having high resolution, which is used to measure the acceleration of a vehicle, may be calibrated to the zero-acceleration value when the vehicle is at a standstill. This state may be detected in a simple manner by the wheel speed zero and/or the turned off engine.

[0012] As a further example for realizing the device and the method according to the present invention, the calibration of a yaw-rate sensor should be mentioned in this context. By calculating the angular motion of the vehicle on the basis of two GPS satellite navigation signals emitted by two different transmitters, a nominal state of the yaw-rate sensor is able to be determined.

[0013] The present invention also makes it easy to calibrate a voltage meter as it is used, for example, to measure the electrical system voltage of a motor vehicle or in the measuring of the resistance of a fuel-level measuring device, by using a bandgap-voltage source as reference. These reference sources are very accurate and are well suited for integration in an ASIC.

[0014] In a particularly advantageous variant of the present invention, the measured value is adjusted in a force sensor in the unloaded state of the force sensor, since this state may be reliably detected in a relatively easy manner.

[0015] If the force sensor is used to detect the weight of a vehicle occupant, the unloaded state may be determined simply on the basis of seat occupancy. If the seat is not occupied, the corresponding force sensor must be in an unloaded state. The seat occupancy may be detected with the aid of a passenger compartment camera in an uncomplicated manner.

[0016] If the force sensor is a belt-force sensor, the unloaded state may easily be recognized via the belt-buckle state, since an operative belt-force sensor in all likelihood is in an unloaded state when the belt buckle is open. The belt-buckle state may be monitored, for instance, with the aid of a belt-buckle contact integrated in the belt buckle. Thus, the calibration procedure could be initiated in an easy manner by an appropriate signal of the belt-buckle contact.

[0017] The calibration of a sensor as proposed by the present invention should be repeated at frequent time intervals in order to ensure reliable measuring results in the long term. Only then will it be possible to utilize the data acquired by a force sensor for detecting the weight of a vehicle occupant and by a belt-force sensor for an adaptive triggering of the corresponding restraining means.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] As already mentioned earlier, there are various possibilities of embodying and further refining the teaching of the present invention in an advantageous manner. In this context, reference is made to the subordinate claims to claims 1 and 7, on the one hand, and to the following description of an exemplary embodiment of the present invention in light of the drawing, on the one hand.

[0019] The sole FIGURE illustrates a possible arrangement of a belt-force sensor, which includes a device for calibration according to the present invention in a motor vehicle.

DESCRIPTION OF THE EXEMPLARY EMBODIMENT

[0020] The single FIGURE shows a vehicle seat 1 and the associated safety belt 2 with belt-mount fixtures 3 and a belt buckle 4, as they may be disposed in a motor vehicle (not shown here).

[0021] In the exemplary embodiment described here, vehicle seat 1 is provided with a sensor mat as it has been described in the introduction. However, other means for weight determination may be provided, too. To supplement the data regarding the seat occupancy acquired with the aid of the sensor mat, a belt-force sensor 5 is provided, which is integrated in belt buckle 4 in this case. However, the belt-force sensor may also be laterally arranged on the belt buckle, for example, between the belt buckle and its anchoring in the vehicle.

[0022] In addition, belt buckle 4 is provided with a belt-buckle contact 6, via which the respective belt-buckle state is ascertained, i.e., whether the belt buckle is open or closed. Since belt-force sensor 5 is in an unloaded state when belt buckle 4 is open, this state may be reliably detected with the aid of belt-buckle contact 6.

[0023] The signal from belt-buckle switch 6, together with the signal from belt-force sensor 5, is conveyed to a control device (not shown further here), which may likewise be arranged in vehicle seat 1. The two signals are then evaluated within the framework of a passenger classification.

[0024] At certain time intervals, synchronized with belt-force sensor 5, the belt-force sensor is calibrated in order to counteract age-related falsification of the measured value, for example, by a temporally variable offset of the sensor characteristic curve. For this purpose, the measured value of belt-force sensor 5 is set to zero when belt-force sensor 5 is in the unloaded state, that is, when belt buckle 4 is open.

[0025] Should this calibration procedure be interrupted by closing of belt buckle 4, the calibration procedure must be repeated as soon as possible. The time interval between the individual calibration procedures should therefore be selected shorter than absolutely necessary.

[0026] Finally, it should be pointed out again that the calibration of the belt-force sensor will not necessarily have to take place in the unloaded state, by setting the zero value. Essential for the present invention is only that the belt-force sensor is in any defined state during the calibration, so that the measured value corresponding to this defined state may be set. A prerequisite for this are means for detecting such a defined state. It is not necessarily be the case that these be realized in the form of a belt-buckle contact. 

What is claimed is:
 1. A device for calibrating a sensor (5) characterized by means for detecting at least one defined state of the sensor (5).
 2. A device for calibrating a force sensor as recited in claim 1, characterized by means for detecting the unloaded state of the force sensor.
 3. A device for calibrating a force sensor or a force-sensor array for detecting the weight of a vehicle occupant as recited in claim 2, wherein the means for detecting the unloaded state of the force sensor or the force-sensor array include means for detecting the seat occupancy.
 4. The device as recited in claim 3, wherein the means for detecting the seat occupancy include a passenger compartment camera.
 5. The device for calibrating a belt-force sensor as recited in claim 2, wherein the means for detecting the unloaded state of the belt-force sensor (5) include means for detecting the belt-buckle state.
 6. The device as recited in claim 5, wherein the means for detecting the belt-buckle state include at least one belt-buckle contact (6).
 7. A method for calibrating a sensor, wherein it is detected with the aid of suitable means when the sensor is in a defined state and that a measured-value adjustment of the sensor corresponding to this state is implemented only when the sensor is in a defined state.
 7. A method for calibrating a force sensor as recited in claim 7, wherein it is detected with the aid of suitable means when the force sensor is in an unloaded state.
 8. A method for calibrating a force sensor or a force-sensor array for detecting the weight of a vehicle occupant as recited in claim 8, wherein the seat occupancy is ascertained and that it is detected on the basis of the seat occupancy when the force sensor or the force-sensor array is in an unloaded state.
 9. The method as recited in claim 9, wherein the seat occupancy is detected with the aid of a passenger-compartment camera.
 10. A method for calibrating a belt-force sensor as recited in claim 8, wherein the state of the belt buckle (4) is ascertained and that it is detected on the basis of the state of the belt buckle (4) when the belt-force sensor (5) is in an unloaded state.
 11. The method as recited in claim 11, wherein the state of the belt buckle (4) is ascertained with the aid of a belt-buckle contact (6).
 13. The use of a device as recited in one of the claims 1 through 6 or a method as recited in one of the claims 7 through 12 for the repeated calibration of a sensor, preferably at regular time intervals. 